core: Add MacOS fallbacks to look for binaries

* src/posix-util.c (find_executable): New.
(walk_path_str): Factored out from walk_path.
(walk_path): Replaced by find_executable.
(_gpgme_get_gpg_path, _gpgme_get_gpgconf_path): Use find_executable.

--
This should help to locate GnuPG on MacOS systems where
it is not part of the PATH environment variable and
should reduce the need to have fixed path known
at GPGME compile time.

mailvelope/issue699

src/posix-util.c

@@ -79,27 +79,18 @@ _gpgme_set_override_inst_dir (const char *dir)
   return 0;
 }
 
-
-/* Find an executable program PGM along the envvar PATH.  */
+/* Find an executable program in the colon seperated paths. */
 static char *
-walk_path (const char *pgm)
+walk_path_str (const char *path_str, const char *pgm)
 {
-  const char *orig_path, *path, *s;
+  const char *path, *s;
   char *fname, *p;
 
-#ifdef FIXED_SEARCH_PATH
-  orig_path = FIXED_SEARCH_PATH;
-#else
-  orig_path = getenv ("PATH");
-  if (!orig_path)
-    orig_path = "/bin:/usr/bin";
-#endif
-
-  fname = malloc (strlen (orig_path) + 1 + strlen (pgm) + 1);
+  fname = malloc (strlen (path_str) + 1 + strlen (pgm) + 1);
   if (!fname)
     return NULL;
 
-  path = orig_path;
+  path = path_str;
   for (;;)
     {
       for (s=path, p=fname; *s && *s != ':'; s++, p++)
@@ -114,12 +105,50 @@ walk_path (const char *pgm)
       path = s + 1;
     }
 
+  free (fname);
+  return NULL;
+}
+
+/* Find an executable program PGM. */
+static char *
+find_executable (const char *pgm)
+{
+  const char *orig_path;
+  char *ret;
+
+#ifdef FIXED_SEARCH_PATH
+  orig_path = FIXED_SEARCH_PATH;
+#else
+  orig_path = getenv ("PATH");
+  if (!orig_path)
+    orig_path = "/bin:/usr/bin";
+#endif
+  ret = walk_path_str (orig_path, pgm);
+
+  if (!ret)
+    {
       _gpgme_debug (NULL, DEBUG_ENGINE, -1, NULL, NULL, NULL,
                     "gpgme-walk_path: '%s' not found in '%s'",
                     pgm, orig_path);
+    }
+#ifdef __APPLE__
+  /* On apple, especially when started through gpgme-json via
+     the browser interface we should look into some additional
+     fallback paths. */
+  const char *additional_path = "/usr/local/bin:/usr/local/MacGPG2/bin";
+  if (!ret)
+    {
+      ret = walk_path_str (additional_path, pgm);
+    }
+  if (!ret)
+    {
+      _gpgme_debug (NULL, DEBUG_ENGINE, -1, NULL, NULL, NULL,
+                    "gpgme-walk_path: '%s' not found in '%s'",
+                    pgm, additional_path);
+    }
+#endif
 
-  free (fname);
-  return NULL;
+  return ret;
 }
 
 
@@ -130,7 +159,7 @@ walk_path (const char *pgm)
 char *
 _gpgme_get_gpg_path (void)
 {
-  return walk_path (default_gpg_name? default_gpg_name : "gpg");
+  return find_executable (default_gpg_name? default_gpg_name : "gpg");
 }
 
 
@@ -139,7 +168,7 @@ _gpgme_get_gpg_path (void)
 char *
 _gpgme_get_gpgconf_path (void)
 {
-  return walk_path (default_gpgconf_name? default_gpgconf_name : "gpgconf");
+  return find_executable (default_gpgconf_name? default_gpgconf_name : "gpgconf");
 }
 
 /* See w32-util.c */